Exemplary of such a composite is a carbon fiber/carbon matrix brake disc made by depositing a carbon matrix on a carbon fiber substrate of the invention, the fibrous material of the substrate being carbonized to reinforce the carbon matrix with carbon fibers. Deposition of carbon on the substrate is effected by in situ cracking of a carbon bearing gas (hereinafter referred to as carbon vapor deposition, abbreviated "CVD" or carbon vapor infiltration, abbreviated "CVI", as these terms are used interchangeably for purposes of the present invention) or by repeatedly impregnating the substrate with a carbon bearing resin and thereafter charring such resin or a combination of such methods to densify the carbon matrix on the carbonized substrate. The invention is not directed to formation of the carbon matrix or densification of the carbon fiber substrate, but rather to the substrate, its preparation, and subsequent densification in known manner to provide a carbon fiber reinforced composite, especially one suitable for use as a friction disc in a brake or clutch.
Such friction disc preforms are commonly prepared from virgin polyacrylonitrile (PAN) fiber which, particularly if CVD is to be employed, is preferably in an oxidized condition which facilitates subsequent carbonization. Oxidized PAN fiber (which may hereinafter be referred to as "OPF") is available commercially in various forms, including tows, yarns, woven and non-woven fabrics, knit fabrics and felts. Layers of such materials may be mechanically united prior to CVD, for example, by needlepunching. Suitable 12k PAN tows may be obtained from Zoltek of Bridgeton, Mo., and RKT of Muir of Ord, Scotland. As used herein the term "tow" is used to refer to a continuous strand of continuous filaments. As used herein the term "yarn" is used to refer to a continuous strand of continuous or staple fibers or blends of these; thus the term "yarn" encompasses tow.
In certain known processes for the manufacture of carbon fiber reinforced friction discs, such as brake discs employed on aircraft, as an example only, annuli are cut out of a parallel-sided sheet of PAN fiber material of the requisite thickness or a plurality of annuli of lesser thickness are stacked and joined by needlepunching to form a substrate of the desired thickness. As shown in FIG. 7, this procedure results in considerable wastage of expensive continuous filament PAN sheet 70 because of the relatively large amount of offcut material that is generated when annuli are removed therefrom which cannot be reprocessed to continuous filament form to make a new continuous filament sheet. Additional expensive PAN sheet material is wasted during subsequent CVD densification of such an annulus in known manner. During such CVD densification the pores of the annulus nearest its surface experience the fastest rate of deposition and thus become of reduced size thereby limiting the rate of deposition, particularly within the central portion of the annulus. According to known CVD practice, following CVD to partial density in known manner, the partially densified discs are removed from the CVD furnace and subjected to a machining operation in which the outermost portion of the annulus is removed thereby removing the outermost material whose pores have been occluded due to deposition of carbon or other matrix therein. The partially densified machined annulus is then returned to a CVD furnace for further CVD densification. This process is repeated until the desired final density is obtained. At the end of each CVD densification furnace cycle a little more of the outermost portion of the annulus is removed by machining, thereby reopening the passageways for the CVD gasses into the central portion of the core. This known practice results in considerable wastage of expensive PAN sheet. The term "partial density" and related forms as used herein means not up to the minimum density specified to exist for a given product at the conclusion of all CVD densification cycles.
One approach to reduce wastage of expensive PAN sheet material in the production of preforms to be used in production of discs for aircraft braking systems is described in EP publication 0 232 059 A2 to Smith. According to Smith, a shaped filamentary structure is prepared in the following manner: needlepunching a unidirectional layer of filaments to provide a degree of dimensional stability; cutting a plurality of segments from the layer of needlepunched material; assembling a plurality of such segments in side-by-side contiguous relationship to produce a filamentary layer of the required structural shape; superposing at least one similar layer on the first layer; and needlepunching the superposed layers to assemble and join the segments. The purpose of cutting segments from the dimensionally stable layer and assembling those segments side-by-side to produce a structural shape is to reduce wastage of the material because it is possible to lay out the segmental shapes to enable maximum use of filamentary sheet material. Nevertheless, offcut waste is generated and there remains a need to efficiently recycle such waste.
As described in GB 2 012 671B to Sherrin et al., offcut PAN sheet material from the cutting of annuli can be recycled by chopping it into staple fibers and thereafter forming a new fabric sheet by needlepunching a layer of carded (recycled) staple fibers to a substantially unidirectional array of continuous filaments extending transversely to the average direction of the carded staple fibers, from which reformed sheet material annuli are again cut out and the off-cut fabric sheet material may again be recycled in like manner. Annuli cut out of the sheet may be stacked to form a substrate for an all carbon brake disc. The stacked annuli may be needlepunched to hold them together.
Thus, there remains a need for reducing the amount of expensive virgin OPF needed to make a CVD friction disc both in forming the annular preform and its subsequent CVD densification. There remains a need for a method for recycling offcut fibrous waste without having to card it and combine it with longitudinal arrays of continuous filaments as is required according to GB 2 012 671B.
According to an aspect of the invention there is provided a fabric sheet having first and second faces, the fabric sheet comprising at least one layer of a web of non-woven fibers prepared by recycling fibers selected from the group consisting of PAN fibers including OPF, carbon fibers, graphite fibers, ceramic fibers, precursors of carbon fibers and precursor of ceramic fibers, and mixtures of these, the web having substantially uniform randomness in direction and an mean fiber length of between 10 and 25 millimeters (0.4 and 1.0 inches) when tested according to ASTM D 1440-77 (1982) (Array Method) using the double bank sorter, Suter-Webb Duplex Cotton Fiber Sorter.
According to another aspect of the invention there is provided a method of making a sheet of fibrous material comprising:
a) opening offcut fibrous material made from PAN fibers including OPF, carbon fibers, graphite fibers, ceramic fibers, precursors of carbon fibers and precursor of ceramic fibers, and mixtures of these, to form discrete staple fibers having an mean length of between about 10 and about 25 millimeters (0.4 and 1.0 inches) when tested according to ASTM D 1440; and
b) airlaying said opened fibers into a web and pre-needlepunching the web to a thickness of from about 5 to about 11 millimeters and a fiber volume of from about 7 to 14 percent.
The resultant airlaid web of the invention may be joined to one or more layers of tow, woven, braided, or knit material or mixtures of these to form a unitary sheet by needlepunching. Multiple layers of such airlaid web may be stacked directly upon one another and joined by needlepunching to either face of a core preform that has been formed in known manner, e.g. such as in EP publication 0 232 059 A2, or GB 2 012 671B or superposed layers of tow that have been needlepunched into a unitary sheet. These outermost layers may be progressively sacrificed by machining away during subsequent multiple furnace cycle CVD densification. Also, layers of such airlaid web may be used to form a new core preform by stacking such airlaid web, preferably in a repetitive pattern, most preferably in alternating manner, with additional layers of tow, woven, braided, knit material, and mixtures of these and forming them into a unitary sheet by needlepunching. Such preforms may be CVD densified in known manner thereby binding together the needlepunched layers with a matrix selected from carbon, ceramic, precursor of carbon, precursor of ceramic, and mixtures of these. The airlaid recycled fiber web of the invention may be prepared at lower cost than carding and thereafter joining the carded layer to a layer of continuous filaments by needlepunching as in GB 2 012 671B. Also, the properties of the airlaid web are preferred for both sacrificial layers and as part of the core because of their random fiber orientation which exists even prior to needlepunching. In contrast, in a carded layer, needlepunching is the only way to create Z-direction fibers. As used herein, the term "Z-direction" means inclined out of the plane defined by the web, and for a curved web inclined toward the radial direction of the web rather than the tangential or the circumferential direction of the web.
The resultant recycled fiber web of the invention may be employed in the manufacture of friction discs, gasketing, high temperature insulation, felting, carbonized paper and possibly other applications.
The above and other features and advantages of the invention will become more apparent when considered in light of the following description of preferred embodiments of the invention in conjunction with the accompanying drawings which also form a part of the specification. In the drawings like numbers are used to refer to like parts or features.